The present invention relates to a method of measuring elemental workpiece shapes of any desired spatial orientation on a coordinate measuring machine having a measuring probe head. The expression "elemental shapes" as used herein is to be understood as comprehending inter alia such elemental workpiece features as bores, corners, spherical or flat surfaces, etc.
More particularly, the invention relates to a so-called "scanning" method in which the probe pin of a probe head remains in continuous contact with the workpiece surface in the course of scanning displacement. Such scanning methods make it possible rapidly to record a large number of measurement points which describe the shape of the workpiece. For this purpose, a so-called measuring probe head is required, i.e., a probe head which has measured-value transmitters which provide signals that are proportional to the magnitude of probe deflection in the respective coordinate directions.
Scanning methods which use measuring probe heads are in themselves already known and are described, for example, in German Patent 2,921,166 and in U.S. Pat. No. 4,769,763.
In the known method and coordinate-measuring machine of German Patent 2,921,166, and after the probe has contacted the workpiece, the probe head is moved or guided at constant speed along and in the axial direction of a first coordinate, the so-called primary coordinate. At the same time, the probe is kept in continuous contact with the workpiece by a readjustment procedure pursuant to a signal from a measured-value transmitter in the probe head, wherein said signal corresponds to instantaneous deflection of the probe in a second direction which is perpendicular to the primary-coordinate axis. As soon as the speed of readjustment displacement becomes greater than the controlled advance in the primary coordinate, the two axes are interchanged. In this way, the probe head automatically follows contours on the workpiece, and these contours need not have been previously known.
The method described in U.S. Pat. No. 4,769,763 also permits automatic scanning of unknown workpiece contours, by predetermining the absolute value of the scanning speed and the desired probe deflection, and by using measured deflection of the probe in a process of continuous recalculation of the direction of scanning displacement tangential to the surface of the workpiece.
It is characteristic of both known methods that the measured-value transmitters of the probe head are combined in a control circuit and that the direction of advance is continuously readjusted in accordance with these signals. For this reason, both methods are relatively slow.
From International Patent Application WO 90/07097, a scanning method is known by which a large number of measurement points on two-dimensional workpiece geometries are to be recorded rapidly. For this purpose, the probe pin of the coordinate measuring machine is mounted to a so-called "measuring rotate-swing joint" or "articulating head", which affords selective rotary positioning of the probe pin about each of two orthogonally related axes. The coordinate measuring machine controls articulation of this joint or head with constant speed over a path of simple geometry, for example, a straight line or a circle, at the same time, the probe pin is applied substantially perpendicular to the direction of travel, with constant force against the workpiece, and is driven in a spiral oscillating movement. Only very few formed features of given predetermined geometry can be measured with this method. Furthermore, it is not possible to scan workpiece features in any desired spatial orientation, since the articulating joint already requires a certain preorientation of the elemental shapes.